This invention relates to a high voltage semiconductor device and more particularly to a semiconductor device having an SOI (Silicon On Insulator) substrate.
In the field of power electronics, semiconductor power devices are widely and actively used in order to attain high controllability and energy saving. The types of the power devices used in this field are changed from the generation of current-controlled type power devices such as thyristors, GTOs (gate turn-off thyristors), and bipolar transistors to the generation of voltage-controlled type power devices using MOS gates, for example, power MOSFETs, and IGBTs (Insulated Gate Bipolar Transistors). The advent of the MOS power device has made the high-frequency switching possible, significantly enhanced the controllability of the device and reduced the size thereof.
As the structure of the IGBT, a four-layered structure of p.sup.+ n.sup.- pn.sup.+ obtained by additionally forming a p.sup.+ -type layer on the drain n (n.sup.-) layer of an n-channel MOSFET is known. There are two types of structures, a vertical type structure in which the p.sup.+ -type collector (or the drain of the IGBT) layer is formed under the MOSFET and a lateral type structure in which it is formed in parallel to the MOSFET on the substrate on which the MOSFET is formed. In the case of lateral type, an SOI (Silicon On Insulator) substrate can be used and it is suitable for a smart power device having a control circuit and the like integrated therein.
In the operation of the n-channel IGBT, an inversion layer (channel) is formed in the p layer lying directly under the gate electrode by applying a positive voltage higher than the threshold voltage between the gate and emitter (or source of the IGBT) and then electrons are started to be injected from the n.sup.+ layer lying directly under the emitter electrode into the n.sup.- drain (or base of the IGBT) layer. The electrons act as minority carriers in the p.sup.+ n.sup.- p transistor so as to cause holes to be introduced from the p.sup.+ collector (drain) layer, thereby making it possible to perform the conductivity modulation and lower the saturation voltage between the collector and emitter.
Thus, the IGBT has two features including the feature of low ON-voltage of the bipolar transistor and the feature of the gate voltage control of the MOS device, and in the design of the IGBT for simultaneously attaining the above two features, it is important to set the trade-off between the low ON-voltage characteristic and the switching time into the optimum condition.
More specifically, in the IGBT, since the conductivity modulation is caused and the ON-resistance is lowered by injecting holes which are minority carriers into the n.sup.- -type base layer, a current flows in the device while the stored holes are being discharged even if the gate is turned OFF to interrupt injection of the electrons, and as a result, the switching speed thereof becomes lower than that of the MOSFET. Various proposals have been made to enhance the switching speed.